If you previously had the book in a collection, it may be confused.
I get this when I pass an Amazon book through calibre to sanitize it, if it was in a collection beforehand I have to delete the book, add the old copy, delete THAT one from collections, and delete it, before I can add my sanitized copy and get on with things.
As far as I can tell, this only happens with books purchased from Amazon and
not converted. (because if you convert it, you're really making a new book.
)